Biological control is the use of living organisms to maintain pest populations below damaging levels. Biocontrol agents include a wide variety of life forms, including vertebrates, invertebrates, fungi, and microorganisms. These beneficial species are common in most natural communities and, although their presence is often unnoticed, they help maintain the “balance of nature” by regulating the density of their host or prey population. Insect species often become “pests” when this ecological balance is disrupted by natural events or human intervention.

Natural enemies of arthropods fall into three major categories: predators, parasitoids, and pathogens. These are the primary groups used in biological control of insects and mites. Most parasites and pathogens, and many predators, are highly specialized and attack a limited number of closely related pest species.

Predators

Predators kill and feed a lot of prey during their lifetimes. Many species of amphibians, birds, mammals, and reptiles prey extensively on insects. Predatory beetles, flies, lacewings, true bug, and wasps feed on various pest insects or mites. Most spiders feed entirely on insects and predatory mites feed primarily on pest spider mites.

A parasite is an organism that lives and feeds in or on a host. Insect parasites can develop on the inside or outside of the host’s body. Often only the immature stage of the parasite feeds on the host. However, adult females of certain parasites (such as many wasps that attack scales and whiteflies) feed on and kill their hosts, providing an easily overlooked but important source of biological control in addition to the host mortality caused by parasitism. True parasites (e.g., fleas and ticks) do not typically kill their hosts. Species useful in biological control kill their hosts; they are more precisely called “parasitoids.”

Most parasitic insects are either flies (Order Diptera) or wasps (Order Hymenoptera). Parasitic wasps occur in over three dozen Hymenoptera families. For example, Aphidiinae (a subfamily of Braconidae) attack aphids. Trichogrammatidae parasitize insect eggs. Aphelinidae, Encyrtidae, Eulophidae, and Ichneumonidae are other groups that parasitize insect pests. It’s important to note that these tiny to medium-sized wasps are incapable of stinging people. The most common parasitic flies are the typically hairy Tachinidae. Adult tachinids often resemble house flies. Their larvae are maggots that feed inside the host.

Pathogens

Natural enemy pathogens are microorganisms including certain bacteria, fungi, nematodes, protozoa, and viruses that can infect and kill the host. Populations of some aphids, caterpillars, mites, and other invertebrates are sometimes drastically reduced by naturally occurring pathogens, usually under conditions such as prolonged high humidity or dense pestpopulations. In addition to a naturally occurring disease outbreak (epizootic), some beneficial pathogens are commercially available as biological or microbial pesticides. These include Bacillus thuringiensis or Bt, entomopathogenic nematodes, and granulosis viruses.

1) Bt gene is transferred from the Bacillus into corn. 2) European corn borer feeds on the corn plant and ingests the protein encoded by the Bt gene. 3) The Bt protein penetrates and collapses the cells lining the gut and the insect dies

A great benefit of biological control is its relative safety for human health and the environment, compared to widespread use of broad-spectrum pesticides.

It’s important to preserve existing natural enemies by choosing cultural, mechanical, or selective chemical controls that do not harm beneficial species (only about 1% of all insects and mites are harmful). Most pests are attacked by multiple species of natural enemies and their conservation is the primary way to successfully use biological control. Judiciouspesticide use (e.g., selective, timing), and habitat manipulation (planting a variety of species that flower at different times to provide natural enemies with nectar, pollen, and shelter throughout the growing season) are key conservation strategies.